Axial-flow compressor



Patented Oct. 31 1950- UNITED STATES. PATENT OFFICE Claims.

My invention relates to compressors particularly of the axial-flow type.

An object of the invention is to provide a means of obtaining a maximumpressure rise through the compressor with a high eificiency. Otherobjects will appear from the description, drawings and claims.

The above objects are accomplished b the means illustrated in theaccompanying drawings in which- Figure 1 is an axial section through thecompressor;

Figure 2 is a development of two of the stages showing the type of bladesection and their pitch setting;

Figure 3 is a section of a blade taken along line 33 in Figure 1;

Figure 4 is a vector diagram of the flow shown in relation to twoimpeller blades;

Figure 5 is a vector diagram of the flow relative to the blades ofFigure 4 for two different axial velocities and the same peripheralvelocity;

Figure 6 is a vector'diagram for the flow of fluid shown in relation tothe type of blades of this invention;

Figure '7 is a vector diagram of the fluid flow relative to the bladesof Figure 6 for two different axial velocities and the same peripheralvelocity; and

Figure 8 is a development of several stages including the stages shownin Figure 2.

Figure 9 shows the vector diagramsfor the first two stages.

Figure 10 shows the vector diagrams for the last stage.

I have found by theory and verified by experiment that, as a conditionfor maximum pressure rise in a stage, the increment of peripheralvelocity added by the impeller, commonly called AC, should beequal tothe peripheral speed u at the point on the radius corresponding to themean value of the square of the relative fluid velocity. To achieve thisvalue the stator and impeller blades require a large angle of pitch anda large maximum ordinate of the mean camber line.

Figure 1 shows a compressor I having a development of the stages asshown in Figure 2. It will be observed that each of the blades 2 of thefirst impeller I1 has a blade section set with such a large pitch thatthe angle 6 between the plane of rotation 4 and the tangent to theundersurface of the section is greater than 60 degrees and the zero liftline 6 of the airfoil sectlon' exceeds 90 degrees. This line gives thedirection of the flow for zero lift on the section, and consequently itmay at first glance appear that the blade set at such a large anglewould pump the air toward the inlet of the compressor rather than theexit as it should. Actually it will be in the correct direction, that istoward the compressor exit even when the blades are set with a stillgreater value of pitch such as 01 for the blade 2a shown dotted. If therotor blades give any rotation to the flow, as they of course do, thestators will convert this rotary flow into an axial one. By thisarrangement an axial flow is begun and immediately grows to a largemagnitude reducing the angle of attack between the blade and the truerelative flow direction. In order to do this the stator blades are givena high camber.

The axial-flow compressor incorporating the arrangement of the blades ofFigure 2 is shown in Figure 1. Fluid enters the compressor inlet 8 andflows through the annular passage III to the exit l2. Preferably boththe stator and impeller blades have boundary layer control slots intheir upper surfaces to enable the blades to operate at large angles ofattack a relative to the resultant flow vector w and large liftcoeflicients as described in my U. S. Patent No. 2,344,835 issued March21, 1944. It will not be further described here except to remark thatthe ducts I4 interconnect the interiors of the blades of spaced stagesso that the pressure difference of the stages causes a flow through theblade slots and this flow controls the boundary layer. Thus, forexample, the impeller blades l8 are connected by tube It to impellerblades 20. Stator blades are connected in like manner, for example blade28 with blade 26 via tube Ida. In this arrangement the upstream bladehas a discharge slot and the downstream blade has an induction slot.

The impeller blades are 2, and I5 to 20, while the stator blades are 22to 30.

An induction blade is shown inFigure 3 where the induction slot is 22.The blade section has the mean camber line 24 with the maximum ordinate26 above the subtending chord OB.

In order to achieve the large value AC imparted to the fluid in theperipheral direction the value of the maximum ordinate 26 should begreater than 5% of the length of the subtendlng chord OB and can be aslarge as 60%. The value used will depend on the type of machine butusually it will be of the order of 20%.

The zero lift line is found as the line drawn through the trailing edgeB and the midpoint P 3 of mean camber line. The pressure distributioncurve is 33.

Figures 4 and 5 illustrate the mode of operation of the conventionalcompressor. Figure 4 shows an impeller blade in relation to the'airvelocity vectors. The axial velocity is Cm, the peripheral component isit giving the resultant velocity w. This is the air velocity relative tothe blade. The vector for the air leaving the stage is V. These vectorsare shown for two conditions in Figure 5, one for an axial velocity ofCm and another for an axial velocity of Cam. In the first case thechange in peripheral velocity is AC, being the peripheral diflerence invectors w; and V1.

When the axial velocity is increased to cum the leaving velocity V2still has the same direction as V1. Hence AC: is less than Acul and thepressure rise of the compressor has decreased.

With the compressor of the present invention, Figures 6 and 7, theresultant entering velocity is w; as shown in Figure 6 for a flrst case.The leaving velocity V: is axial indirection. For a second case theaxial velocity becomes Cmz. and the leaving velocity V: is still axial.Hence ACu is equal to u and remains constant. As remarked earlier thisvalue of AC can be shown to offer the optimum pressure conditions.

It will thus be clear that the present invention makes possible anincrease in volume pumped while maintaining the pressure. This is a veryimportant characteristic.

It is usually desirable to obtain as large a pressure rise as possiblefrom a given number of stages of an axial-flow compressor. To this endit is desirable to operate the machine with a tip peripheral speed ashigh as possible. However, the local velocity of relative flow must beless than the speed of sound or a compressibility shock occurs whichlimits the performance of the machine. This shock occurs whenever thelocal velocity attains substantially the velocity of sound in the localmedium. The acoustic velocity is a function of the absolute temperatureof the fluid, an increased temperature resulting in an increase ofacoustic velocity.

Fluid passing over the upper surface of a blade increases in localvelocity as is well known in aerodynamics. When this local velocityreaches the speed of sound in the local fluid, a compressibility shockwill occur and seriously increase the blade resistance. To avoid theshock the ratio of local velocity to the acoustic velocity should beless than unity. The ratio is commonly called the Mach Number.

Since the fluid is coldest at entrance, the tip speed of the impellerblades is set by the speed of sound in the fluid at entrance. The tipspeed can, however, be increased by giving the air an initial rotationat entrance in the same direction as the blade rotation. Thisexpediency, however, reduces the pressure rise available from themachine. It has the advantage, however, of permitting the directconnection of the impellers to the shaft of the gas turbine which canoperate at a greater tip speed because of the high temperature of themotive gas, giving a high value to the velocity of sound.

In the compressor the temperature rises due to the compression so thatthe blades of the later stages could be operated at a higher tip speedthan the blades of the first stage. Where the stages. are all on oneshaft this is not practical. This invention discloses a means, however,of utilizing the increasing fluid temperature.

An initial vortex in the direction of rotation is induced at theentrance of the compressor by stator stage 30, Figures 1 and 8, toobtain a high tip speed relative to the compressor case but low relativeto the fluid. This vortex is then dissipated in successive incrementsfrom stage to stage, preferably in the first half of the stages. Where agreat many stages are used and the overall compression ratio is to behigh, the last stages may be even subjected to a vortex of counterrotation.

To dissipate the initial vortex, I increase the camber of successivestator or guide vane stages. The camber increases the local velocity onthe upper surfaces of the blades but since the temperature has beenincreased by the compression,

- the local velocity of flow can still be well below the local velocityof sound.

The cambers oi the impeller blades are also increased from stage tostage to obtain greater lift coeflicients. This also results in greaterlocal velocity on the upper surface of the blade but again it is belowthe local velocity of sound because of the temperature rise,

in some .of the downstream stages I utilize some of the increasedtemperature by arranging the stator blades to direct the fluid flowcounter to the direction of rotation of the impellers. This increasesthe velocity of the fluid relative to the impeller and gives a greaterpressure rise while at the same time not exceeding a Mach Number of one.

If the procedure of increasing the camber is followed to keep the localMach Number Just under unity, and if a large number of stages are used,the cambers of the downstream stages will become very large even thoughthe cambers of the blades of the initial stages are quite small. Theblades of real high camber will require boundary layer control but theearlier stages may not.

Figure 8 shows the development of the blades at a point somewhat outfrom the midpoint of the radius. This figure shows that the camber ofthe blades of successive stages is increased in the downstreamdirection. It also shows that succeeding stages. deflect the fluid so asto give successively less component of velocity in the peripheraldirection. This is indicated by the decreasing angle 5 between the zerolift lines 8 and the axis 32 of the machine. This figure shows that inthe fourth stator stage $4 the angle has changed to a negative angle(-5) whereas in the first stage it was positive.

It will also be observed from Figure 8 that the camber oi the blades ofsucceeding stages has increased in the downstream direction.

Figure 9 shows the vector diagrams for the first two stages where theair is discharged from the impeller in a substantially axial direction.This axial direction of discharge may be maintained for several otherstages but since the size of the compressor is usually of greatsignificance the vector diagrams are changed to the type shown for stage05 S, Fig. 10, which may be typical of several neighboring stages.

The notation used places a prime on the symbol for the leaving velocity.The absolute velocities are indicated by C with proper sufllx while 70the relative velocities are given by v with proper sufllx. Thus theinlet velocity to the first stator is 00 and the leaving vector is Co.The rotational velocity is it which combined with Co gives an the vectorrelative to impeller I1. It will be observed that Co has a component inthe direction of rotation. The, fluid leaves Irwith the vector in normalto the plane of rotation. The air enters the stator C2 and leaves withC2. It is to be noted that this vector has a smaller component in ,thedirection of rotation of I2. In the compressor of Fig. 1 this componentreverses direction at an intermediate stage and actuallyis directedagainst the direction of rotation. Stage iistypical of such stages.

Fluid from I; is discharged toward S5 along C5 and leaves S5 along C5.This vector has a component in the plane of rotation of impeller I5counter to its direction of rotation. When,

' rality of stages ahead of the stage where the vector from the statorchanges the direction of its horizontal component the temperature hasrisen and accordingly the velocity of sound in the fluid has increasedin magnitude. Thus the increased velocity component such as as can beaccommodated without cempressibility shock.

To recapitulate, I have disclosed a compressor which can operate at thebest condition for producing a high compression ratio. That is, it canbe operated to give the fluid a peripheral change in velocity equal tothe peripheral speed of the blade. This is many times the value providedby conventional axial-flow compressors. To accomplish this, thecompressor has blade sections of large arching of the mean camber lineand the blades are set at very large pitch angles.

For operating at very large peripheral speeds relative to the case,while yet not at large speeds relative to the fluid pumped, statorblades are placed ahead of the first impeller to induce a vortex of thesame rotational direction as the impeller. This somewhat reduces themaximum pressure ratio of the front stages but the loss is more thanregained because the later stages can be operated at large speedsrelative to the air because of the rising temperature of thecompressedfluid and because the stators can be arranged to dissipate the initialvortex and may even advantageously introduce a vortex rotating counterto the impellers. By these procedures I use the terms stator blades andguide vanes interchangeably to indicate the vanes which direct .thefluid from one impeller to another. I do not intend to imply that statorblades or guide vanes are necessarily stationary. They might,

for instance, be rotating oppositely to theimpellers.

While'I have illustrated a specific form of this invention it is to beunderstood that I do not intend to limit myself to this exact form butintend to claim my invention broadly as indicated by the appendedclaims.

What is claimed is:

1. In combination in a compressor, a case, hub means mounted in saidcase in spaced relation thereto forming therewith an annular flowpassage within having an inlet and an exit and conveying a fluid flow,said hub means being mounted for rotation about an axis, a plurality ofstages of impeller blade; mounted on said hub means to impel said fluidflow, a plurality of stages of stator blades supported in said case witha stator stage ahead of each said impeller stage, the blades of a saidstator stage aheadof an upstream impeller stage being angularlypositioned to defiect said fluid flow in the direction of the rotationof the adjacent downstream impeller stage, a said stator stage ahead ofa said downstream impeller stage having vanes angularly positioned todeflect said fluid flow against the direction of rotation of theadjacent downstream impeller stage, said annular passage between saidupstream and downstream impeller stages having substantiallycontinuously decreasing cross sectional areas in the downstream di-'rection to increase the velocity of the flow from said downstream statorstages against said adjacentdownstream impeller stage, a plurality ofsaid impeller and stator stages being positioned the number of stagesfor a given pressure ratio can be greatly reduced. For instance, an18-stage compressor can be reduced to six stages.

quire boundary layer control to compel the flow to follow the bladesurfaces.

.1 do not intend to limit the invention to blades requiring boundarylayer control for high lift. In the case of the initial vortex, forinstance. the first stages might have low cambers well below the needfor boundary layer control. The camber could then be increasedsuccessively to values at the rear stages which would just escape theneed for boundary layer control. Such a machine would not equal thepressure performance Where the early stages are highly cambered and setat large pitch angles as described.

I have described one type of boundary layer control but I do not intendto limit myself to this. In particular, I intend to claim any type ofslot in the blade surface supplied with air by any means.

I use the terms impeller and rotors interchangeably to designate theblade and hub structure to ahead of the said downstream impeller stagefirst subjected to a counter-rotation inflow so as to increase the fluidtemperature by compression to raise the velocity of sound in the fluidto a value higher than the local fluid velocity on the blades of saidimpeller stage subjected to said counter flow from the adjacent upstreamstator stage. said impeller stage subjected to counter-rotation inflowhaving cambered blades with slots therein, and means inducing a flow offluid through said slots for compelling the said flow to cling to thesurface of said blade.

2. In combination, in an axial-flow compressor guide vanes ahead of thefirst impeller, said guide vanes being positioned to cause rotation ofthe fluid in the direction of rotation of said first impeller, a.downstream said guide vane stage having vanes angularly positioned todirect said fluid flow in counter-rotation to the direction of rotationof an adjacent downstream said impeller, the first said impellersubjected to a counterrotation inflow being preceded by a plurality ofsaid impellers adapted to produce a substantial temperature rise in saidfluid ahead of said impeller subjected to counter-rotating fluid, theblades of successive impellers in the downstream direction having bladesections of increasing maximum height of the mean camber are above thesubtending chord to successively increase the temperature and pressureof said successive impellers, said temperature rise increasing thevelocity of sound in the fluid to a value higher than the local fluidvelocity on the blades of said impeller subjected to said counterrotation flow.

3. In combination in an axial flow compressor, a hub means supported forrotation about an axis, a plurality of blades supported on said hubmeans forming a plurality of impeller; spaced along said axis, aplurality of guide vanes forming a plurality of guide vane stagesinterposed between said impellers to direct a fluid flow from oneimpeller to another, an upstream group of said guide vane stagesdisposed successively in the downstream direction being adapted todirect said fluid flow in the direction of rotation of said impellerswith a successively smaller component of peripheral velocity for eachsaid successive stage, and a downstream group of successive said guidevane stages being adapted to direct said fluid flow counter to thedirection of rotation of said impellers adjacent thereto, the first saidimpeller subjected to a counter-rotation inflow being preceded by aplurality of said impellers adapted to produce a substantial temperaturerise in said fluid ahead of said first impeller subjected tocounter-rotating fluid, the blades of successive said impellers in thedownstream direction having blade sections of increasing maximum heightof the mean camber line above the subtending chord the fluid to a valuehigher than the local fluid velocity on the blades of said impellersubjected to counter-rotation in flow and on the blades of saidsuccessive impellers, said blades having slots in their upper surfaces,and means to induce a flow of fluid therethrough to control the flow onthe blade surfaces.

4. In combination in a compressor having a main flow of fluidtherethrough, a plurality of stages of impeller blades mounted forrotation about an axis, a plurality of stages 01 stator guide vanessupported with a said stage ahead of and adjacent each said impellerstage, said vanes of a stator stage ahead of an upstream impeller stagebeing angularly positioned to deflect said fluid cent downstreamimpeller stage, a plurality of said impeller and stator stages beingpositioned rotation inflow having cambered blades with slots therein,and means to induce a flow of fluid through said slots for compellingthe said main flow'to cling to the surface of each said blade.

5. In combination in a compressor having a main flow of fluidtherethrough, a plurality of stages of impeller blades mounted forrotation about an axis, and a plurality of stages of stator vanessupported with a stage ahead of each said impeller stage, said vanes ofthe stator stage ahead of an upstream impeller stage being angularlypositioned to deflect said fluid flow in the direction of rotation ofthe adjacent downstream impeller stage, a said stator stage ahead of adownstream said impeller stage having vanes angularly positioned todeflect said fluid flow against the direction of rotation of theadjacent downstream impeller stage, a plurality of said impeller andstator stages being positioned ahead of the said downstream impellerstage first subjected to a counter-rotation inflow so as to increase thefluid temperature by compression to raise the velocity of sound in thefluid to a value substantially higher than the local fluid velocity onthe blades of the said impeller stage subjected to said counter inflowfrom said adjacent upstream stator stage.

EDWARD A. STALKER.

REFERENCES CITED The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS Number Name Date 936,114 Gardner Oct. 5, 19091,463,110 Worthen July 24, 1923 2,234,733 Jendrassik Mar. 11, 19412,314,058 Stalker Mar. 16, 1943 2,344,835 Stalker Mar. 21, 1944 FOREIGNPATENTS Number Country Date 8,741 Great Britain 1906

